1. Field of the Invention
The present invention relates to a temperature monitoring device for a heating cylinder of an injection molding machine.
2. Description of the Related Art
In general, an injection device of an injection molding machine comprises a heating cylinder and a nozzle. The heating cylinder is used to heat and melt a molding resin and maintain the molten state of the resin. The nozzle is attached to the distal end of the heating cylinder and serves to inject the resin melted in the heating cylinder into the cavity of a mold. The temperatures of the heating cylinder and the nozzle are controlled without deviation to a preset temperature such as to maintain the molten state of the resin, by using temperature measuring unit such as a thermocouple or heating unit such as a heater. The necessary temperature of the heating cylinder for the maintenance of the molten state of the resin is determined to be, for example, 200 to 450° C., depending on the type of the resin. A stable molding operation can be continuously performed by maintaining this necessary temperature.
If an operator touches the heating cylinder or the nozzle at such a high temperature as to molt the resin, during the molding operation or in a stop mode, he/she may possibly get burned. In some cases, moreover, the resin heated and melted in the heating cylinder may be ejected from the nozzle, and the operator may be burned by touching the ejected resin.
Conventionally, a protective cover as a unit configured to covert a heating cylinder to avoid the possibility of a burn or the like described above is attached to the heating cylinder (Japanese Patent JPH9-1613A and JP2000-108187A). During a molding operation, however, a resin melted in the heating cylinder is injected into the cavity of a mold through the tip of a nozzle. Therefore, the operation must be performed with an injection device advanced by the drive of a nozzle-touch motor or a hydraulic device and with the nozzle on the distal end of the heating cylinder pressed against the mold on a stationary platen of an injection molding machine. Accordingly, part of the heating cylinder or the nozzle is not covered by the protective cover.
In some cases, therefore, openable safety doors are attached around a heating cylinder or a nozzle. Some openable safety doors are designed to entirely cover a heating cylinder and a nozzle or an injection device. Other openable safety doors are designed to cover a heating cylinder and a nozzle only partially.
In a technique disclosed in Japanese Patent JP2007-225019A, a machine is provided with an electromagnetic lock as a unit configured to lock openable safety doors. The operation of this machine can be stopped to release the electromagnetic lock by opening the safety doors. According to this technique, the electromagnetic lock can be released to allow the safety doors to open when the machine is stopped. If the machine is stopped so that the safety doors can be opened, however, the temperature of the heating cylinder is not always reduced to a safe level.
Specifically, even if temperature control by heating devices is suspended as the operation is stopped, the current temperature of the heating cylinder continues to be high for a while and will not be reduced to a safe level such that an operator can touch the cylinder. Further, it is necessary to start the temperature control of the heating cylinder by the heating devices before the start of the operation so that the resin in the heating cylinder is melted in advance. In some cases, therefore, the heating cylinder may stay hot even when the machine is stopped.
Japanese Patent JP2003-191308A discloses a technique in which a display device displays an indication to the effect that a heating cylinder is hot or an operation command to lock safety doors. However, there is no disclosure to the effect that the safety doors should be locked based on the current temperature of the heating cylinder.
In the prior art techniques, as described above, the safety doors can be opened and closed without considering the current temperature of the heating cylinder or the nozzle, so that the possibility of an operator getting burned by touching the heating cylinder cannot be eliminated.